1. Field of the Invention
The invention relates generally to an improved process for the production of anhydrous ethanol, and in particular to methods to improve regeneration of molecular sieve materials used in ethanol dehydration processes.
2. Description of the Background Art
Anhydrous ethanol is widely used in industry as a solvent in the synthesis of paints, pharmaceuticals and intermediaries, cosmetics, perfumes, and other products, for example. Anhydrous ethanol is also an important component in alternative fuels, such as gasohol, when combined with gasoline and other fossil fuel distillate components to make fuel for motor vehicles and other combustion engine applications. Anhydrous ethanol can also be used as an important oxygenic additive in lead-free gasoline.
However, even small amounts of water in ethanol can lead to the formation of unwanted products under the conditions present in typical combustion engine combustion chambers, or other industrial synthetic processes. For example, 4.4% (wt) water and 95.6% (wt) ethanol will form an azeotrope under conditions greater than 78° C. temperature and exceeding 1.0 bar pressure, well below the threshold combustion conditions in an internal combustion engine. An azeotrope is a mixture having a constant boiling temperature and so is difficult to separate the components by distillation. Therefore, even a small amount of water contamination in anhydrous ethanol is extremely undesirable. Thus, the process of dehydrating or drying ethanol is a valuable process for producing anhydrous ethanol for use as a fuel or as a solvent in many industrial processes.
Traditional distillation to obtain anhydrous ethanol is a costly process requiring high amounts of energy to obtain pure anhydrous ethanol. Other processes that can be used to obtain anhydrous ethanol include azeotropic distillation, extractive distillation, and salt rectification. However, these processes still involve high energy distillation and its associated high energy costs to obtain anhydrous ethanol, resulting in a more expensive anhydrous ethanol product.
Adsorption purification of ethanol is a process requiring less energy than distillation processes to obtain anhydrous ethanol. Various methods using adsorption to various adsorbent matrices under various temperature and pressure conditions are used to obtain anhydrous ethanol without distillation. One method used for production of motor fuel grade ethanol uses a concentration swing to release adsorption of ethanol from a paper matrix. Another method uses a pressure or vacuum swing to increase adsorption of materials to a matrix, adsorbed material is then desorbed with a depressurization of the adsorption chamber. However, these methods still require large amounts of energy to effect the concentration or pressure swings to elute the anhydrous ethanol from the adsorbent, or require additional amounts of ethanol to be cycled through to maximize the dehydration of the ethanol product.
One adsorbent method of ethanol dehydration uses natural-derived particles as an adsorbent bed material to produce anhydrous ethanol. Examples of natural adsorbent bed materials include corn grits or wheat starch particles. One example of commercially available wheat starch particles is available under the trade name EnviroStrip (Archer Daniels Midland Co., Decatur, Ill., USA). The mechanism of adsorption of water by corn grits and Envirostrip starch particles involves hydrogen bonding of the water molecules with the hydroxyl groups on the starch chains (Rebar et al., 1984). Both types of starch chains, amylose and amylopectin, interact with water molecules in this manner. However, the amylopectin structures also physically trap water molecules in the matrix of chain branches (Rebar et al., Biotechnology and Bioengineering, 26, 513-517 (1984)). When the water molecules are trapped this way, some nearby —OH groups become unavailable for hydrogen bonding.
In spite of the advantages of using corn grits as a readily available and inexpensive adsorption dehydration matrix for ethanol/water mixtures, there are limitations of corn grits used as an adsorbent matrix. Corn grits may be regenerated by flushing with CO2 gas, but generally have a limited working life in an ethanol dehydration process. Among the problems associated with use of corn grits (or wheat starch adsorbent particles) for ethanol dehydrating beds is that such natural products are fragile, tend to collapse, tend to agglomerate, and rapidly lose their capacity for water adsorption.
Another method used for ethanol dehydration is based on non-organic zeolite type materials that act as a molecular sieve beds. Unlike natural bed adsorbents, molecular sieve beds act as absorbents, where primarily, the water is differentially retained in the bed by virtue of being included in the pores of the bed matrix instead of being adsorbed onto surface features.
Molecular sieve beds require a different type of regeneration than natural adsorbent beds. The molecular sieve absorbents are regenerated in a process that uses anhydrous ethanol in combination with a “pressure swing” technique. In this technique the molecular sieve bed is regenerated by passing previously dehydrated ethanol vapor through the bed at a substantially lower pressure (i.e., a vacuum) than was used for loading the bed for the dehydration step. There are both high pressure and low pressure molecular sieve systems, but the regeneration of each involves use of a relative vacuum along with anhydrous ethanol in the regeneration step.
In one example high pressure system, the feed ethanol entering the bed is loaded at a temperature of 148.9° C. (300° F.) and a pressure of 473.7 kPa (68.7 psia). The anhydrous ethanol used to regenerate the column is loaded at the same temperature and a pressure of 13.8-20.7 kPa (2.0-3.0 psia). In another example high pressure system, the feed ethanol entering the bed is loaded at a temperature of 148.9° C. at a pressure of about 386.1 kPa (56 psia). The anhydrous ethanol used to regenerate the column is loaded at the same temperature and a pressure of 13.8-20.7 kPa (2-3 psia).
In one example low pressure system, the ethanol feed entering the column is loaded at a temperature of 93.3-115.6° C. (200-240° F.), preferably 104.4° C. (220° F.) at a pressure of 114.4 kPa (˜16.6 psia). The anhydrous ethanol used to regenerate the column is loaded at the same temperature range and at a pressure of less than 13.8 kPa (2.0 psia). In another low pressure system, the ethanol feed entering the column is loaded at a temperature of 115.6° C. (240° F.) and a pressure of 137.9 kPa (20.0 psia). The anhydrous ethanol used to regenerate the column is loaded at the same temperature and a pressure of 17.2 kPa (2.5 psia). In yet another low pressure system, the ethanol feed entering the column is loaded at a temperature of 115.6° C. (240° F.) and pressure of 170.3 kPa (24.7 psia). The anhydrous ethanol used to regenerate the column is loaded at a temperature of 104.4° C. (220° F.) and a pressure of 10.3 kPa (1.5 psia).
There are significant drawbacks using a pressure swing technique, including high energy cost incident to drawing a substantial vacuum and waste of a portion of already dehydrated ethanol for regenerating a bed whose very purpose is to make the dehydrated ethanol.
There is therefore, a need for improved processes for producing anhydrous ethanol that reduce the energy requirements and waste incident to regenerating molecular sieve beds used for multiple dehydrations. The present teaching provides materials and methods that meet these and other needs and is applicable not only to dehydrating ethanol, but to dehydrating any organic solvent for which a molecular sieve bed is used for dehydration.